1. Field of the Invention
The present invention relates to an image fixing apparatus used in an image forming apparatus such as a copying machine, printer, facsimile, etc.
2. Description of the Related Art
In a typical conventional fixing apparatus, a recording medium on which an image has been formed is passed through a nip formed between a cylindrical fixing roller and a cylindrical pressure roller so that the recording medium is heated by a halogen heater serving as heating means disposed in a hollow of the fixing roller thereby fixing the image on the recording medium. The temperature of the fixing roller is detected by a thermistor temperature sensor (hereinafter referred to simply as a thermistor) disposed in contact with the circumferential surface of the fixing roller.
During a continuous fixing process for a plurality of recording media, the temperature of the fixing roller is controlled for example as follows. The temperature at the circumferential surface of the fixing roller is detected using the thermistor during the continuous fixing process for the plurality of recording media. If the temperature detected by the thermistor becomes lower than a recording medium feeding interval switching temperature which is predetermined without taking account the type and the size of recording media and without taking account the effects of changes in the environmental conditions, the recording medium feeding interval at which the recording media are fed to the nip is increased (that is, the number of recording media fed per unit time is reduced). This temperature controlling method is disclosed for example in Japanese Patent Laid-Open No. 54-80135.
The above-described technique is based on the idea that good fixing capabilities are maintained by controlling the recording medium feeding interval regardless of a reduction in the temperature of the fixing roller.
However, in the method of controlling the fixing temperature according to Japanese Patent Laid-Open No. 54-80135, the interval of feeding recording media to the nip is switched when the temperature detected by the thermistor becomes equal to the feeding interval switching temperature which is determined without taking account the type of the recording media and the changes in the environment conditions, it is difficult to maintain the surface temperature of the fixing roller within the allowable range of fixing temperature during the entire fixing process for a user-specified number of recording media.
When recording media have a large weight per unit area (hereinafter such a recording medium will also be called thick paper), the recording media absorb a large amount of heat from the fixing roller during the fixing process. Conversely, when recording media have a small weight per unit area (hereinafter such a recording medium will also be called thin paper), the recording media absorb a small amount of heat from the fixing roller during the fixing process.
The amount of heat that a recording medium absorbs from the fixing roller during the fixing process varies depending on the environments in which the apparatus is used, no matter which type of recording medium is employed.
Therefore, even if the number of recording media fed per unit time is reduced when the temperature of the fixing roller has dropped to the feeding interval switching temperature, there is a possibility that, if there are a larger number of remaining recording media to be fixed, the temperature of the fixing roller will further drop below the minimum allowable fixing temperature.
The reduction in the recording medium feeding speed results in a reduction in the processing speed of the apparatus. From the viewpoint of productivity, it is desirable that the reduction in the recording medium feeding speed be as small as possible.
Thus, it is desired to improve the conventional fixing sequence so that it will be capable of accommodating various types of recording media and handling changes in the environmental conditions.
In view of the above, an object of the present invention is to provide an image fixing apparatus capable of entirely fixing a specified number of recording media without generating fixing defects.
It is another object of the present invention to provide an image fixing apparatus capable of utilizing the full fixing capacity.
According to an aspect of the invention, to achieve the above objects, there is provide an image fixing apparatus comprising: a heating member controlled to keep a first temperature; a back-up member cooperable with the heating member to form a nip through which a recording medium passes; feeding rate control means for controlling the rate of feeding recording media per unit time in such a manner that the rate of feeding recording media per unit time is reduced when the temperature of the heating member has dropped to a second temperature lower than the first temperature; and temperature setting means for setting the second temperature in accordance with the rate at which the temperature of the heating member changes when a plurality of recording media are continuously passed through the nip.
According to another aspect of the present invention, there is provided an image fixing apparatus comprising: a heating member controlled to keep a first temperature; a backup member cooperable with the heating member to form a nip through which a recording medium passes; and feeding rate control means for controlling the rate of feeding recording media per unit time during a process of continuously fixing a specified number of recording media, in such a manner that when the temperature of the heating member has dropped to a second temperature lower than the first temperature, the feeding rate per unit time for the remaining recording media is determined depending on the number of remaining recording media.
These and other aspects, features, and objects of the present invention will become more apparent from the following detailed description of preferred embodiments as illustrated in the accompanying drawings.